(1) Field of the Invention
The present invention relates to a stabilized styrene polymer which is improved especially in weathering resistance.
(2) Description of the Related Art
Owing to the excellent moldability and mechanical and electrical properties, styrene polymers have been used for a variety of commercial products such as household electrical appliances, OA apparatus, automobile parts, etc. Styrene polymers, however, generally have the defect that they tend to be deteriorated by light, in other words, they are poor in weathering resistance.
In order to prevent such deterioration by light, it has been generally practiced to blend various types of weathering stabilizer such as benzophenone, benzotriazole, benzoate, nickel chelate and hindered amine compounds in the styrene polymer.
However, the styrene polymers containing such weathering stabilizer can still hardly be said as quite satisfactory in weathering resistance. Also, the fact is to be noted that, as often practiced recently, various types of pigment are blended in styrene polymer for the purpose of improving external appearance of the molded products, but such blending of pigment tends to adversely affect the weathering resistance of the products.
Among said various types of weathering stabilizers, the hindered piperidine compounds represented by the formula: ##STR2## wherein R.sub.1 is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms,
R.sub.2 is a hydrogen atom or an alkyl group having 1 to 12 carbon atoms, and PA1 R.sub.3 and R.sub.4 are each independently an alkyl group having 1 to 4 carbon atoms
are known as an effective light stabilizer.
For instance, U.S. Pat. No. 4,500,622 discloses the compound of the formula (I) wherein R.sub.1 and R.sub.2 are each hydrogen atom and R.sub.3 and R.sub.4 are each methyl, and it is stated that the use of this compound in combination with a phenolic antioxidant is conductive to the improvement of weathering resistance of certain types of polymers such as polypropylene. However, no satisfactory improving effect is provided for the weathering resistance of styrene polymers even by the combined use of such hindered piperidine compound and phenolic compound.
The present inventors had previously found that the joint use of a hindered piperidine compound of the formula (I), a phenolic antioxidant and a sulfur-containing antioxidant could produce a prominent effect for the stabilization of resins such as polypropylene, polyurethane resin and ABS resin, and applied for a patent under U.S. patent application Ser. No. 754,582. Combined use of said hindered piperidine compound, phenolic antioxidant and sulfur-containing antioxidant could provide a better effect for the stabilization of said resins than the composition disclosed in said U.S. Pat. No. 4,500,622 but was still unable to realize a satisfactory improvement of weathering resistance of styrene polymers blended with a pigment.